For the plastic working of airplane and automobile parts, marine products such as boats, building materials, kitchen fitments, and bathroom fitments such as bath tubs, press-working using metal dies has been generally used. However, with methods using metal dies and presses, the plant is large and a large installation space is required, and plant costs and die-making costs are extremely high. Also, the forming of complex shapes is difficult and requires high-level process technology and finishing skill. Furthermore, because press operation produces noise and vibration it has an adverse affect on the environment, and safety measures have also been problematic.
One known alternative is the spinning method, but because this method involves molding a sheet by pressing it onto a rotating mold it has had the fatal shortcoming that it is only possible to form moldings whose cross-section is a circular cylindrical or conical shape.
In this connection, in Japanese Unexamined Patent Publication No. 7-132329, one of the present inventors has proposed a progressive sheet-forming method and apparatus. In this prior art, a barlike pressing member having a spherical end part is brought into contact with the underside of a sheet; a moving pressing member having a spherical pressing part is brought into contact with the other side (the upper side) of the sheet; and, with the periphery of the sheet held with a fixed holding force by a screw-type holding tool, the moving pressing member is moved around the barlike pressing member in correspondence with the cross-sectional shape of a product to be formed while the holding tool is moved in the thickness direction of the sheet by a spring-type cushion.
However, with this prior art, although the forming of simple diverging shapes such as conical shapes and pyramid shapes is possible, the forming of shapes wherein a bottom and a side wall part (trunk part) join at a sharp corner is not possible, and in particular, when the dimensions of a product are large, because the framelike holding tool supporting the sheet tends to incline and drop, there have been problems of forming becoming impossible or the accuracy of the formed shape deteriorating. Consequently the forming of products, typified by bath tubs and sinks, which have a large bottom area, of which furthermore the bottom profile shape may be irregular, and which have a high side wall part continuing from a bottom part, or which have a step at an intermediate level in a side wall, has been impossible.
Also, because this prior art is simple stretch-forming, carried out with the periphery of the sheet clamped, when the forming of a side wall which is vertical or at a near-vertical angle .alpha. is carried out, a blank of a length l.sub.0 in the horizontal state extends to a length l.sub.1, and along with this the sheet thickness decreases from t.sub.0 to t.sub.1 (t.sub.1 =t.sub.0 sin.alpha.), so that for example a sheet thickness of 2 mm decreases to 0.17 mm, and thus the percentage sheet thickness decrease is high. Consequently there has been the problem that, depending on the material of the sheet and the sheet thickness, cracks may form in the side wall and local deformation may occur so that forming is almost impossible, and even if forming is possible there is a marked fall in strength.
There has also been the problem that when a hard sheet such as a stainless steel sheet is formed by this prior art method it is difficult to control spring-back, and formability and shape accuracy have consequently tended to be poor. And it has been a further problem that in cases where the product has not a simple flat flange but a bent-back flange it is not possible to carry out forming of this flange part.